1. Field of the Invention
The present invention relates to spherical metal-oxide powder particles, which are used as finely ground raw material and filler materials in mineral, ceramic and refractory construction, technical and auxiliary materials, as well as for polishing and grinding agents, and to a process for producing such powder particles, as well as to the application of this process for treating ceramic and refractory residual materials.
2. The Prior Art
According to the prior art, ceramic and refractory residual materials are treated by sorting, crushing and classifying. If the purity is high enough, recycling may take place, otherwise, only disposal as waste in a waste dump is possible.
Special problems are caused in connection with mechanical treatment by residual materials in which there is a fusion product of metallic aluminum and its alloying components which are present. These fusion products result from melting and casting factory installations within the aluminum industry, for example such as metal-contaminated refractory furnace refuse, or refuse from ceramic liquid-metal filters or aluminun dross. This waste material, following meltdown with salts, which permits partial recovery of the aluminum, and subsequent leaching out of the salts of the melt with water, again lead to environmental problems. This also leads to sludge residues which, because of their content of aluminum nitride and metallic components of aluminum alloys, release ammonia and hydrogen. Furthermore, also residual substances of ceramic and metal composite materials (e.g. aluminum reinforced by SiC-particles or Al.sub.2 O.sub.3 fibers, or Al-bonded Al.sub.2 O.sub.3, silicon nitride-bonded SiC, or Si-bonded SiC) pose problems in the mechanical treatment.
In connection with mineral, ceramic and refractory building and technical materials, the mechanical strength is important. It is influenced by the size and shape of any defective areas in the structure. Therefore, with construction and technical materials which are produced from powders, the objective is to minimize the size and form of such defects by using powder particles with a particle size as fine as possible, and preferably in the spherical form. Since the powders are, as a rule, mixed with water for processing, ball-shaped powder particles with a narrow distribution of the particle size are ideal particularly with respect to the rheological behavior.
With refractory applications, thermal stability, heat insulation stability, resistance to temperature changes and resistance to corrosion are also required in addition to mechanical strength. Metal-oxide materials based on aluminum silicates (such as mullite, sillimanite, cyanite, bauxite), or magnesium aluminate (spinel), and calcium aluminates (refractory concrete) are used in such applications as well. For their manufacture, powder particles with a hollow interior structure (hollow spheres in the ideal case) are particularly advantageous with respect to heat insulation capability.
According to the state of the art, it is possible to produce metal-oxide powder particles containing or forming mullite, spinel, or calcium aluminates, by finely grinding suitable raw materials, mixing them, and pelletizing or pressing them into shaped blanks of centimeter size. Subsequently, sintering and melting can be carried out in rotary tubular kilns, shaft or tunnel kilns, or in arc furnaces. This results in particulate intermediate products of &gt;1 mm, from which powder particles are obtained by subsequent grinding and classifying. Powder particles produced in this way always have a splintery grain form. Moreover, the limits of technical possibilities are reached when grinding and classifying powders in the range of below 5 .mu.m.
Wet chemical precipitation techniques (e.g. sol gel) are known by which it is possible also to produce spherically shaped, metal-oxide composite powders of less than 5 .mu.m in diameter. Following drying, however, such powders still contain a substantial amount of chemically bonded water and, therefore, have to be converted into the oxide form by heating (calcining). However, when calcining powder particles smaller than 5 .mu.m in diameter, it is not possible to avoid agglomerations and sinterings, which change the shape of the grain.
A process for recycling aluminum oxide from aluminum oxide waste and refuse portions is described in U.S. Pat. No. 5,424,260 (1), whereby non-metallic products such as 40-75% by weight Al.sub.2 O.sub.3, 5-20% by weight MgO, and 2-15% by weight SiO.sub.2 are collected, and possibly also trace elements in the form of titanium, copper, zinc, potassium, sulphur, calcium and sodium. Depending on the composition of the refuse, a temperature between 1800.degree. F. and 3500.degree. F. is selected in order to convert the components of the mixture into a molten phase. From the molten phase, either amorphous, noncrystalline structures are obtained by rapid cooling, or crystalline structures are obtained by very gradual cooling. Following grinding, the products can be used as abrasives.
It is known, furthermore, to produce spherically shaped mineral particles by atomizing melts, forming hollow spheres in this process as well. A drawback of the process technique lies in the fact that the hollow spheres produced thereby have a wide grain size distribution in the millimeter-range (e.g. 0.5 to 3 mm), and that the wall of the sphere is so thin that it breaks easily. Such hollow particles require much preparation water when processing them into cement-bonded construction materials.
WO 91/10629 (2) and WO 92/10629 (3) describe processes for treating residual materials in the aluminum industry such as aluminum wastes, in which sintered refractory spinel or mullite products are manufactured by adding magnesium oxide or silicon oxide, and are then crushed to a particle size of 5 to 50 .mu.m.
JP 63185803 (4) describes a process for producing spherically shaped metal-oxide composite powders with &gt;85% particles in the diameter range of 0.02 to 0.30 .mu.m. Subsequently, powdery metal alloys (Mg--Al or Al--Si or Mg--Si) are charged into a reaction chamber with oxygen and evaporated there at 1200.degree. C. Following cooling, spherical powders are formed consisting of spinel (MgO.cndot.Al.sub.2 O.sub.3) or mullite (3Al.sub.2 O.sub.3 .cndot.2SiO.sub.2) or forsterite (2MgO.cndot.SiO.sub.2), which are suitable for producing pigments, fillers, sintering materials, and as sintering aids.
Condensed silicon oxides are known from the carbothermal production of silicon metal and silicon alloys. The particles of such silicon oxides are spherical and have a diameter in the range of 0.04 to 0.5.mu., a specific BET-surface between 6.2 and 18.5 m.sup.2 /g, as well as a density of 2.21 to 3.13 g/cm.sup.3. (P. Aitcin et al: Ceramic Bulletin, 63, 1984, pp. 1487-1491 (5)).
DE 4,241,625 (6) and EP 601,453 (7) describe a process for producing sintering-active, largely spherical aluminum oxide with an average particle diameter of less than 1 .mu.m, preferably of less than 0.5 .mu.m. In this process, an aluminum carrier such as metallic aluminum or aluminum oxide is evaporated in a furnace. Following the subsequent oxidation in a gas stream, an aluminum oxide powder is separated using a filter, such powder having a density of 2.5 to 3.97 g/cm.sup.3 and a specific BET surface of 0.5 to 60 m.sup.2 /g .